The invention is concerned with the field of semiconductor technology. It relates in particular to an IGBT (Insulated Gate Bipolar Transistor) according to the preamble of the first claim. Such an IGBT is disclosed for example in the published German Patent Application DE 196 12 516 A1.
In IGBTs according to the prior art, the gate signal in IGBT chips with a minimum surface area of 0.2 cm2 is initially distributed over the periphery of the chip with the aid of a gate runner (see DE 196 12 516 A1). Narrow strips (gate fingers) then carry the signal inside the chip (clearly evident in EP 0 755 076 A2). Both the gate runner and the gate fingers are composed of an Al metalization layer. It is also possible for the signal, proceeding from a gate pad arranged in a corner or centrally, to be distributed over the chip surface via gate fingers (see FIG. 1). The design rule that is usually used states that the spacing x of the gate fingers must fulfill the following condition.   x  ⁢       less than  less than                   τ                  R          ·          c                    
R is the sheet resistance of the polysilicon which carries the signal from the gate finger to the physical gate, c is the MOS capacitance per area of the gate, and xcfx84 is the characteristic switching time of the IGBT, given by xcfx84=RGate.Ctot. RGate is the gate series resistance and Ctot is the total MOS capacitance of the chip. Typical values are c=30 nF/cm2, R=30 ohms and xcfx84=200 nsec. That results in x less than  less than 0.47 cm. If the above condition is fulfilled, thenxe2x80x94independently of the gate series resistancexe2x80x94the voltage distribution in the gate at any time during the switching is even and hence the current density is homogeneous.
However, gate fingers require complex and expensive soldering metalization if cathodal soldering is necessary, and impose high requirements on the passivation. Passivation weaknesses lead to gate-emitter short circuits which are manifested as premature failures and can only be detected by complicated burn-in tests. Similar problems arise in the case of pressure contact-making.
The object of the invention is to specify an IGBT which can be produced in a simple manner yet can be turned on homogeneously.
The essence of the invention, then, is that the gate current in the IGBT-Chip is forwarded, proceeding from the gate terminal, directly via the polysilicon layers of the gate electrodes to the IGBT standard cells, without the use of gate fingers. The gate signal can be fed to the IGBT chip via a gate terminal (gate pad) arranged in a corner according to a first exemplary embodiment or via a central gate terminal in accordance with a second exemplary embodiment.
The invention thus marks a departure that is diametrically opposed to the prevailing view that above a certain chip size, homogeneous turn-on can only be achieved using gate fingers. Rather, the inventors recognized for the first time that the design rules applicable to the homogeneous turn-off of an IGBT differ from those applicable to MOS transistors. The inventors have recognized, in particular, that the maximum dissipated power density is always homogeneous if the plasma distribution is still homogeneous. Particularly at the instant when the MOS current is no longer enough to maintain the external current, a bipolar component has a fundamentally different behavior than a unipolar element such as e.g. a MOS transistor. Said instant is followed by a transition from bipolar current to pure hole current, associated with depletion of the main junction. As soon as this process has concluded, the entire current is carried by holes and the space charge zone is built up. However, the plasma distribution is still very homogeneous laterally. Therefore, the highly inhomogeneous current distribution is also quickly homogenized. At the instant of the maximum power loss, the current distribution is virtually homogeneous. Therefore, the safe operating area is not reduced and the turn-off energy is hardly changed. As a consequence, IGBTs can be constructed without gate fingers, even when a minimum area of 0.2 cm2 that is applicable to gate-fingerless MOS transistors is exceeded.
FIGS. 2a and 2b show an exemplary embodiment of an IGBT chip 1, according to the invention. The illustration shows a first main terminal 3 surrounded by insulation 7 and a gate frame 8. The gate frame 8 is connected to a gate terminal 4, from which e.g. bonding wires may lead to the corresponding housing terminal. In contrast to this, gate fingers 6 are provided in the case of the prior art according to FIGS. 1a and 1b, said gate fingers distributing the gate signal over the chip surface, proceeding from the gate terminal 4.
The advantages of the invention can be seen, in particular, in the fact that
the obviation of the gate fingers enables simple and inexpensive implementation of the soldering metalization,
the simpler technology enables a higher reliability in the pressure contact-making of the chips.